This invention relates to a method and apparatus for improving aircraft safety. More specifically, it relates to an apparatus and method for preventing combustion in aircraft fuel tanks and cargo spaces by supplying inert gas.
Military aircraft have used On-board Inert Gas Generating Systems (OBIGGS) for some years to protect against fuel tank explosions due to undesired phenomena, such as penetration from small arms fire. Military aircraft are not the only aircraft that would benefit from OBIGGS. For example, investigations into the cause of recent air disasters have concluded that unknown sources may be responsible for fuel tank ignition and explosion. Subsequently, OBIGGS has been evaluated as a way to protect commercial aircraft against such fuel tank explosions started by unknown ignition sources.
OBIGGS protects against fuel tank explosions by replacing the potentially explosive fuel/air mixture above the fuel in the tanks (the ullage) with an inert gas (usually nitrogenxe2x80x94N2). The nitrogen is generated by separating oxygen from local, ambient air and pumping the inert product into the tanks.
In previous applications, OBIGGS has proved relatively unreliable, heavy, and costly for both initial acquisition and operation. Furthermore, military aircraft systems often have the strict requirements that derive from military flight profiles that include high-rate descent from high-altitude flight. Applications of OBIGGS to commercial aircraft would benefit by considering the fact that a typical flight profile for the commercial application has a less demanding requirement for the system because commercial aircraft do not descend at such rates.
Thus, there is a need for a gas generation and inerting system that minimizes the quantity and complexity of sub-components (particularly moving parts). The system should also consider a typical commercial aircraft flight profile and take advantage of the reduced descent rates (compared to military aircraft).
The present invention provides an apparatus and method for reducing the possibility of combustion in aircraft fuel tanks by replacing air in the ullage of the fuel tank with an inert gas that has been separated from the engine bleed gas. The apparatus includes an air separation module, a filter, and a heat exchanger. In one preferred embodiment, at least the air separation module, filter, and/or heat exchanger are provided in a modular unit, which may be contained in a single housing. Each modular unit is designed to provide predetermined NEA flow. In order to accommodate different inerting requirements of different size aircraft, multiple modular units may be employed.
The method of the present invention includes displacing the atmosphere in the ullage of a fuel tank with a non-combustible gas. In one preferred embodiment, a high-purity non-combustible gas is introduced to the ullage at a low flow rate when external air is in slow influx to the ullage, and alternately a lower-purity non-combustible gas is introduced at a higher rate when external air is in high influx to the ullage. In one embodiment of the invention the non-combustible gas is generated from aircraft engine bleed air. Preferably, the introduction of high-purity non-combustible gas occurs during take-off and level flight of an airplane and introduction of the lower-purity gas occurs during descent of the airplane. In an alternative embodiment, the system is sized to provide a single purity level and flow rate under all operating conditions, thereby eliminating the need for various control components.